Cordierite ceramic filter and method of producing the same

Abstract: A cordierite ceramic filter is formed by mixing cordierite powders having a porosity of more than 30% as aggregate with raw materials for cordierite generation, poring agents, forming agents and solvents to obtain a ceramic batch, extruding the ceramic batch into a honeycomb shape to obtain a formed body, and firing the formed body. The cordierite ceramic filter has a mean pore size in a range of aggregate mean size × 0.15 ± 5µm, has a porosity of more than 30% and has a compressive strength more than 100 kgf/cm² in a direction parallel to a flow passage. The cordierite ceramic filter has an excellent filter regeneration function, has a high mechanical strength and is light and compact.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a cordierite ceramic filter to be used in high temperatures, and a method of producing the same. Related Art Statement Recently, as porous ceramic filters, various types of porous honeycomb filters have become known, wherein a filtration is performed by passing a fluid such as gas through partition walls of a honeycomb structural body made of cordierite. Since cordierite has an excellent heat resistance as compared with the other materials, the cordierite honeycomb is used as a catalyst carrier of automobiles and so on, which needs an excellent thermal shock resistance. In the known cordierite ceramic filter especially used for collecting fine particles in an exhaust gas supplied from a diesel engine, since a filter regeneration is performed by firing the collected fine particles in the ceramic filter, the cordierite ceramic filter is not used stably for a long time period due to a deterioration or a melt down thereof. In order to eliminate the drawbacks mentioned above, it is disclosed that a filter regeneration is performed by using a reverse pulse air. However, the filter regeneration method mentioned above has various drawbacks such that a filter regeneration efficiency is not sufficient, an apparatus for performing the filter regeneration method has not a mechanical strength sufficient for being assembled in the automobiles, and thus a total system for performing the filter regeneration method becomes larger so as to improve the insufficient efficiency and mechanical strength mentioned above. The known cordierite ceramic filter is produced by mixing raw materials such as talc, kaolin, alumina and so on with forming agents to obtain a ceramic batch, extruding the ceramic batch to obtain a honeycomb structural green body, firing the honeycomb structural green body to obtain a honeycomb structural body, and sealing both ends of the honeycomb structural body in a checker pattern, wherein, if one end of a flow passage is closed, the other end of the flow passage is opened. US-A-4 632 683 corresponding to Japanese Patent Laid-open Publication No. 61-129015 (JP-A-61-129015) discloses a method for improving the filter efficiency such that pores being existent in a surface of a filter partition wall are controlled to satisfy a ratio of (the number of small pores having a diameter of 5-40 µm) : (the number of large pores having a diameter of 40-1000 µm) in a range of 1:5-1:40. However, this method is developed for the filter regeneration by firing but for the filter regeneration by the reverse pulse air. Therefore, even if the filter regeneration by using the reverse pulse air is applied to the filter disclosed in JP-A-61-129015, sufficient filter regeneration cannot be performed. As for a filter to which the filter regeneration by using the reverse pulse air is applied, Japanese Patent Laid-open Publication No. 63-31517 (JP-A-63-31517) discloses a method of producing a ceramic filter by mixing one or more than two kinds of ceramic fine powders selected from the group of cordierite, β-spodumene, aluminum titanate, mullite, zirconia, silicon carbide and silicon nitride with clay or frit as a binder to obtain ceramic batches, and firing the ceramic batches. However, the ceramic filter formed according to the method mentioned above cannot obtain sufficient mechanical strength and cannot perform the filter regeneration by using reverse pulse air. Therefore, the ceramic filter mentioned above cannot be preferably applied for the automobiles which needs a long particle operating time. In order to obtain a ceramic body having an isotropic thermal expansion, Japanese Patent Publication No. 60-2272 (JP-B-60-2272) discloses a method of producing a cordierite body by mixing predetermined synthesized cordierite powders having an undetermined shape and raw materials for cordierite generation to obtain mixtures having a theoretical cordierite composition, forming the mixtures to obtain a formed body, and sintering the formed body. This method is for a catalyst carrier, and thus it is effective to the ceramic body which needs an excellent heat resistivity. However, it is not sufficient for the ceramic filter which needs an excellent trapping efficiency, since it does not disclose a porosity and a particle size of cordierite powders. EP-A-630 677 discloses a method of producing the ceramic filter showing a small pressure loss and a sufficient mechanical strength for a real use by using densified cordierite having a suitable particle size as aggregates. However, according to the method mentioned above, in order to densify the aggregates, it is necessary to fire cordierite powders having high porosity and to add LAS raw materials. Therefore, producing steps increase extraordinarily. EP-A-354721 describes a porous honeycomb filter containing cordierite as a main crystalline phase. This is obtained by mixing raw materials such as talc, kaolin and alumina, extruding the mixture and firing. EP-A-232621 describes honeycomb catalyst carriers of cordierite, having porosity less than 30% and high strength. To produce them, starting materials including fine particles of kaolin and talc are employed. Summary of the invention An object of the present invention is to reduce or eliminate the drawbacks mentioned above and to provide a cordierite ceramic filter and a method of producing the same, which filter can show an excellent filter regeneration function, have a high mechanical strength, and be light and compact. According to the invention, there is provided a cordierite ceramic filter for separating a solid phase from a fluid, as set out in claim 1. According to the invention, there is also provided a method of producing a cordierite ceramic filter, as set out in claim 4. Brief Description of the Drawings Fig. 1 is a cross sectional view showing a typical structure of a cordierite ceramic filter according to the invention; Fig. 2 is a cross sectional view illustrating a typical structure of a conventional cordierite ceramic filter; and Fig. 3 is a schematic view depicting one example of an apparatus for measuring filter characteristics. Description of the Preferred Embodiment Since a ceramic filter according to the invention uses cordierite ceramics as a filter aggregate, the ceramic filter according to the invention can endure a high temperature engine exhaust gas. In the ceramic filter, a number of flow passages are formed by partition walls having a honeycomb shape. Both ends of the ceramic filter are sealed in a checker pattern. If one end of each flow passage is open, the other end of this flow passage is closed. Exhaust gases are introduced from opened flow passages, pass through the partition wall made of porous materials mentioned below, and discharge from the filter through adjacent flow passages. Since the exhaust gases are passed through the partition wall, dust particles included in the exhaust gasses are collected by the partition wall. In the ceramic filter mentioned above, since the partition walls have a honeycomb shape, it is possible to increase filter area per unit volume, which contribute to the particle collection. If the ceramic filter is produced from raw materials for cordierite generation only, pores in the filter are not continuous as shown in Fig. 2, which shows a conventional example. This is because pores are generated due to talc and silica included in the raw materials for cordierite generation, and thus particle shapes of talc and silica remain as they are. If such discontinuous pores remain, a filter regeneration by a reverse pulse air is not performed sufficiently, and thus a pressure loss is increased corresponding to lapse of time. Therefore, it is not possible to obtain a ceramic filter having uniform characteristics. In the ceramic filter according to the invention, in order to increase mechanical strength, use is made of cordierite particles as aggregates, and cordierite particles are cross-linked by raw materials for cordierite generation. Therefore, pores generated between aggregates made of cordierite particles show excellent continuity as shown in Fig. 1, and thus it is easy to regenerate the ceramic filter by using a reverse pulse air. As mentioned above, in the ceramic filter according to the invention, if a porosity of overall ceramic filter is more than 30%, the ceramic filter has a sufficient mechanical strength such as a compressive strength of more than 10 MPa (100 kgf/mm 2 ) in the direction parallel to the flow passages. Moreover, such a ceramic filter improves a filtering rate, decreases pressure loss and has a sufficient operating time for a real use. Contrary to this, if a pore continuity and a porosity increase are made much in excess, mechanical strength is decreased since cross-linkings between aggregates are not sufficient. The inventors performed many examinations, and found the result that, if the mean pore size of the ceramic filter is set in a range of (aggregate size × 0.15) ± 5 µm, it is possible to obtain a ceramic filter having excellent pore continuity and also excellent mechanical strength, which ceramic filter is preferably used for a filter regeneration by using a reverse pulse air. That is to say, if the mean pore size is not less than (aggregate size × 0.15) + 5 µm, dust particles are passed through the partition wall, and thus trapping efficiency is decreased or mechanical strength of the partition wall is decreased. Moreover, if the mean pore size is not more than (aggregate size × 0.15) -5 µm, pressure loss is increased and pore continuity is affected. Therefore, in this case, the filter regeneration property is affected. In the preferred embodiment of the ceramic filter according to the present invention, the thickness of the partition wall is set in a range of 300-2000µm. If the thickness of the partition wall is too small, it is difficult to form a honeycomb structural body. Moreover, there occur various problems such as that mechanical strength is insufficient and particle trapping efficiency becomes worse, and thus it is necessary to perform the filter regenerating operation frequently. Moreover, if a thickness of the partition wall is too large, pressure loss is increased, and filter area per unit volume (hereinafter, called as volume efficiency) is decreased. Open frontal area ratio defined by a ratio of open cell area and total area on a cross section of the ceramic filter influences volume efficiency of the filter, mechanical strength, pressure loss and so on together with outer diameter of the filter, density of the flow passages, thickness of the partition wall and so on. In the ceramic filter according to the present invention, it is preferred to set the open frontal area ratio in a range of 63-71%. Moreover, if use is made of cordierite powders having a porosity of more than 30% as aggregates, pores included in the cordierite powders serve as pores between aggregates in the case of firing shrinkage during firing, and thus pores having excellent continuity can be obtained. Mean particle size of the cordierite powders as aggregates is preferably set in a range of 5-450 µm, and it is also preferred to set this mean particle size to a value less than 20% of a slit width of the extrusion die. Thereby, cordierite powders can be compacted sufficiently in the case of extruding operation, so that formability of the honeycomb body becomes better and mechanical strength is improved. If the mean particle size of the cordierite powder is not less than 20% of slit width of the extrusion die, the honeycomb body is not preferably formed, and in an extreme case, the extrusion die is deformed. Moreover, cordierite is not easy to sinter. Therefore, mechanical strength is not sufficient, if cordierite powders are only fired. In the method of producing a cordierite ceramic filter according to the invention, in order to cross-link the cordierite powders, 5-45 preferably 5-30, parts by weight of raw materials for cordierite generation are added as cross-link agents with respect to 100 parts by weight of aggregates. As for the raw materials for cordierite generation, use is made of mixtures which are obtained by mixing materials selected from talc, kaolin, alumina and silica to provide the same composition as cordierite. It is preferred to use raw materials for cordierite generation such that, if they are fired alone, the fired body has a porosity of more than 35%. Moreover, since raw materials for cordierite generation have excellent thermal expansion properties, heat stable properties and chemical stable properties, they serve effectively as the cross-link agents due to liquid phase reactions, and also serve for improving a pore continuity. However, if an amount of raw materials for cordierite generation is in excess, pores are closed like an ink pot shape. In this case, a continuity between pores is deteriorated, and thus a filter regeneration property is decreased. In the cordierite ceramic filter according to the invention, since a predetermined amount of raw materials for cordierite generation is added as cross-link agents, it is possible to obtain a sintered body having an excellent filter regeneration property and an excellent mechanical strength. Pore-forming agents are used to form pores in the sintered body by oxidizing and vaporizing them in the case of firing. For example, as the pore-forming agents, use is made of carbon powders each having a suitable dimension, polyurethane adhesives and so on. In the method of producing the cordierite ceramic filter according to the present invention , it is necessary to add less than 45, preferably less than 30, parts by weight of the pore-forming agent with respect to 100 parts by weight of the aggregates. Preferably forming agents and solvents are used to support a mixing operation and an extruding operation of the raw materials, and use is made of normal forming agents for use in a ceramic powder extrusion. As for the forming agents, use is made of methylcellulose, diethylene glycol stearate and so on. Kinds and amounts of the forming agents can be selected freely, but it is preferred to add more than 3 parts by weight of total agents with respect to 100 parts by weight of the aggregates. Firing temperature of the ceramic honeycomb formed body is selected in a temperature range above a temperature at which raw materials for cordierite generation start to fire below a melting temperature of cordierite. If the firing temperature is too high, aggregates become glass and thus thermal expansion coefficient is increased. It is preferred to fire in a temperature range of 1350-1430°C. It should be noted that the checker pattern sealing of both ends of the filter is performed anytime before or after firing the ceramic formed body. Hereinafter, the present invention will be explained in detail with reference to actual examples. Example 1 As aggregates, cordierite powders each having compositions and properties shown in Table 1 were prepared. As for raw materials for cordierite generation, raw materials having compositions shown in Table 2 were mixed in a ratio shown in Table 2. As for pore-forming agents, use was made of graphite. Then, the thus prepared aggregates, raw materials for cordierite generation and pore-forming agents were mixed as shown in Table 3 to obtain ceramic batches for extrusion. In this mixing step, 10 parts by weight of methyl-cellulose and 3 parts by weight of diethylene glycol stearate were added as forming agents with respect to 100 parts by weight of the mixtures. Moreover, in this mixing step, water was also added as a solvent. Kind of cordierite powders Composition (wt%) Average particle size(µm) Porosity (%) MgO Al 2 O SiO 2 CaO NaO A 13.7 35.5 50.5 0.1 0.2 65 30 B 13.7 35.5 50.6 0.1 0.1 70 30 C 13.7 35.5 50.5 0.1 0.2 65 25 D 13.7 35.4 50.6 0.1 0.2 66 40 E 13.7 35.4 50.6 0.1 0.2 430 30 F 13.7 35.5 50.5 0.1 0.2 55 30 Kind of raw materials Composition (wt%) Mixing ratio (%) Ig-loss MgO Al 2 O 3 SiO 2 TiO 2 Fe 2 O 3 Talc 5.7 30.7 1.2 60.9 - 1.0 40.0 Kaolin 13.9 - 38.6 45.5 0.8 0.4 25.0 Calcined kaolin 0.1 - 45.1 53.1 0.8 0.4 21.0 Alumina 0.3 - 99.4 - - - 14.0 By using a extrusion die having a dimension of slit width: 330µm and 38 cells per square cm, the thus prepared ceramic batches were extruded to obtain ceramic honeycomb formed bodies each having a dimension of diameter: 130mm and length: 170mm. Then the thus obtained ceramic honeycomb formed bodies were fired at 1400°C to obtain ceramic honeycomb bodies. After the firing, both ends of the ceramic honeycomb body were sealed like checker pattern to obtain filter samples. Then, thickness of partition wall and the number of cells were measured with respect to the thus obtained filter samples. Moreover, porosity measurement by using a boiling method and pore measurement by using a mercury-porosimeter were performed. Moreover, cylindrical samples each having a diameter of 25.4mm and a length of 25.4mm were cut out from the filter samples, and a compressive strength of the cylindrical sample along a direction parallel to the flow passage was measured. Further, a diameter of the aggregate was measured by using scanning electron microscope (SEM). By using a measuring apparatus shown in Fig. 3, property of filter regeneration by using reverse pulse air and pressure loss were measured under the condition of exhaust gas flow rate: 3 m 3 /minute, reverse pulse air pressure: 6 kgf/cm 2 , pulse interval: 0.5 second. Measured results were shown in Table 3. In Table 3, the trapping efficiency was indicated by a ratio of an amount of collected particles in a collection tank 14 for three hours from the start of examination and an amount of generated particles for the same time duration. After three hours, the trapping efficiency value was increased or maintained. Moreover, the property of filter regeneration was indicated by a pressure loss after three hours from the start of examination measured by using a pressure loss measuring apparatus 12 shown Fig. 3. In the property of filter regeneration, excellent property was indicated by a small pressure loss value and worse property was indicated by a large pressure loss value. From the Table 3, it is understood that the filter samples having a porosity of more than 30% and a filter pore size of (aggregate size × 0.15) ± 5 µm show excellent pressure loss and excellent property of filter regeneration. Moreover, the filter samples having a filter pore size of less than (aggregate size × 0.15) - 5µm show a large pressure loss, and thus it is clearly understood that, if such a filter is assembled in an engine, the load on the engine is increased. Further, the filter samples having a filter pore size of more than (aggregate size × 0.15) + 5µm show a low trapping efficiency, and thus it is understood that an effective filter cannot be achieved. Furthermore, the filter samples having a porosity of not more than 30% shows a large pressure loss. Moreover, it is understood that the filter samples showing excellent properties are produced under such a condition that use is made of cordierite powders having a porosity of more than 30% as aggregates, and 5-45, preferably 5-30 parts by weight of raw materials for cordierite generation and less than 45, preferably 30, parts by weight of poring agents are added. If an amount of raw materials for cordierite generation is not more than 5 parts by weight, compressive strength is decreased due to insufficient cross-linkings between aggregates, and also pressure loss is increased. If an amount of raw materials for cordierite generation is not less than 45 parts by weight, compressive strength is increased due to sufficient cross-linking, but a pore discontinuous state shown in Fig. 2 is generated. Therefore, property of filter regeneration becomes worse. Moreover, if an amount of poring agents is not less than 45 parts by weight, porosity is increased, but compressive strength is decreased since cross-linkings between aggregates are affected by an increase of porosity. Example 2 Filter samples were produced according to the same producing method shown in Example 1 except for slit width and cell structure. Kinds and amounts of cordierite powders, amounts of raw materials for cordierite generation and pore-forming agents, and measured properties were shown in Table 4. From the results shown in Table 4, it is understood that the filter samples having a thickness of partition wall of not less than 2000µm show a large pressure loss, and the filter samples having a thickness of partition wall of not more than 300µm show a decrease of compressive strength which does not satisfy a strength level to be assembled in engine. Moreover, if thickness of partition wall of the filter samples is in a range of 300-2000µm, the filter samples having an open frontal area ratio on a cross section of not more than 63% show an excellent trapping efficiency (no data) and a sufficient compressive strength, but their pressure losses are increased to an extent not used in a real use. Further, in this case, if an open frontal area ratio on a cross section is not less than 71%, compressive strength is decreased, and filter effective area becomes smaller. Therefore, such filter having a large open frontal area ratio mentioned above is not also used for a real use. As mentioned above, according to the present invention relating to a cordierite ceramic filter and a method of producing the same, it is possible to obtain a cordierite ceramic filter having excellent trapping efficiency, small pressure loss, light weight and compact characteristics, excellent property of filter regeneration and strength sufficient to assemble in a motorcycle.

Claims: A cordierite ceramic filter for separating a solid phase from a fluid, which is formed of cross-linked ceramic aggregates made of cordierite and has a honeycomb shape comprising partition walls which define flow passages which are sealed in a checker pattern at both ends so that, if one end of one flow passage is closed, the other end of said flow passage is open, said filter having a mean pore size in µm in the range (A x 0.15) ±5 where A is the aggregate mean size in µm, a porosity of more than 30% and a compressive strength of more than 10 MPa (100 kgf/cm 2 ) in a direction parallel to said flow passages. A cordierite ceramic filter according to claim 1, wherein the thickness of said partition walls is in a range of 300-2000µm. A cordierite ceramic filter according to claim 1 or 2, wherein the open frontal area ratio at a cross section of said filter is in the range of 63-71%. A method of producing a cordierite ceramic filter, including the steps of mixing 100 parts by weight of cordierite powder aggregates having a porosity of more than 30%, 5-45 parts by weight of raw materials for cordierite generation as cross-link agents for said aggregates, and less than 45 parts by weight of pore-forming agent to obtain a ceramic batch; extruding said ceramic batch into a honeycomb shape to obtain a shaped body; and firing said shaped body. A method of producing a cordierite ceramic filter according to claim 4, wherein the mean particle size of said cordierite powder aggregates is set to a value less than 20% of a slit width of an extrusion die used for extruding said ceramic batch. A method of producing a cordierite ceramic filter according to claim 4 or 5 wherein both ends of said formed body are sealed in a checker patter before or after said firing step. A method of producing a cordierite ceramic filter according to claim 4, 5 or 6 wherein said raw materials for cordierite generation are a mixture obtained by mixing materials selected from talc, kaolin, alumina and silica, which mixture has the same composition as cordierite.

Inventors: KASAI YOSHIYUKI, KOTANI WATARU, KUMAZAWA KAZUHIKO, KASAI, YOSHIYUKI, KOTANI, WATARU, KUMAZAWA, KAZUHIKO, KUMAZAWA, KAZUHIKO, NKG TAKEDA-KITA-SHATAKU
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1994
Main classification: B01D46
IPC codes: B01J21, B01J37, F01N3, B01D46, B01D39, B01J35, C04B38, C04B35, F01N7